Mr Litvinenko, 43, died in November 2006 after he was poisoned with polonium-210 while drinking tea at a London meeting - allegedly with Lugovoy, who denies all involvement. While attempts to extradite Lugovoy to the UK have been rejected by the Russians, it was thought he might have provided video-link evidence to the inquest in London.

According to Interfax, Lugovoy told the press conference: "I have no hope to get justice in the UK. I finally lost faith in the possibility of an impartial investigation of the case in England. I have to say that I'm out of the coroner's investigation and I will not participate in it."

It has been claimed that Lugovoy and another former KGB agent, Dmitry Kovtun, poisoned Mr Litvinenko at the Millennium Hotel in London's Grosvenor Square.

A previous hearing was told that Mr Litvinenko had been hired by MI6 for a number of years and was working with the Spanish secret service investigating the Russian mafia shortly before his death. He was said to regularly meet with an MI6 handler, named only as Martin, in central London and was paid by both the British and Spanish secret services into a joint bank account he held with his wife.

Last month, coroner Sir Robert Owen ruled that sensitive evidence alleged to expose Mr Litvinenko's ties to MI6 will be examined in secret. His family have urged the Government to reveal the documents but Foreign Secretary William Hague has argued that the disclosure could pose a risk to national security.

The nature of the evidence contained within the files remains unclear but lawyers for the Litvinenko family claim the documents could point towards Russian state involvement.

The inquest is due to formally open on May 1, more than six years after Mr Litvinenko was killed. Sir Robert will hold a hearing on Thursday, in which he will hear applications for anonymity of witnesses and will consider submissions on the inquest timetable.

A spokesman for the inquest said: "It is too early to say whether Mr Lugovoy will be called to give evidence and, if he is, how it would be given."